The amount of digital information that can be transmitted over a communications medium such as copper telephone wires, typically provided as a pair of twisted copper wires, is limited by the inherent bandwidth of the medium. Information to be transmitted digitally is first typically converted into digital pulses. The number of pulses that can be transmitted in one second is limited by the bandwidth of the medium. Limiting the number of pulses that can be transmitted limits the amount of data that can be transmitted. To increase the amount of data that can be transmitted, data encoding schemes were created.
One such scheme is pulse position modulation (PPM). A PPM scheme creates a plurality of symbol positions, with each symbol position corresponding to a predefined value representing the information. Placing a pulse in one of the symbol positions transmits the predefined value. In n-PPM, information is converted into n-bit data, where n is greater than zero. The information is then converted into one of 2.sup.n possible pulse values. Each of the values at which a pulse may be placed is referred to as a "symbol position." The unit indicating the n-bit information formed by the 2.sup.n symbol positions is referred to as a "frame.".
The number of frames that can be transmitted per unit time is related to the bandwidth of the communications medium. For example, the bandwidth of a twisted copper pair is typically no more than 64 kHz, which corresponds to a channel capacity of 64,000 bits per second. Therefore, there can be up to 64,000 frames available. As an example, in a 3-PPM system transmitted over twisted copper pairs, there are eight possible combinations of 3-bit binary data: 000, 001, 010, 011, 100, 101, 110, and 111. Each frame is divided into eight symbol positions, having the values 0, 1, 2, 3, 4, 5, 6, 7 (2.sup.3 =8), which correspond to the 3-bit binary data.
For transmission of the 3-bit data with a PPM scheme, a pulse is placed in the corresponding symbol position. In this manner, 3 bits of information are transmitted by a single pulse, as opposed to a single bit of information represented by a single pulse. Therefore, the amount of data transmitted increases by a factor of 3.
Simply increasing the factor n can increase the amount of data transmitted. However, increasing the factor n increases the number of symbol positions, but the number of frames remains the same because of the limited bandwidth of the channel. Necessarily, the width of the individual pulses decreases. This leads to an increase in the required bandwidth of the communications medium used to transmit the PPM signal. However, communications media have a fixed bandwidth. As stated above, for twisted copper pairs, this is typically about 64 kHz for lengths of about 18,000 feet to 24,000 feet. Once the bandwidth limitation of the communications medium is reached, the individual pulses can occur so closely together that the receiver will not be able to resolve one pulse from another.
One possible solution is to switch to a communications medium with a greater bandwidth capacity. However, media with greater bandwidth are typically more expensive than twisted copper pairs and also require more complex transmitters and receivers.
Thus, there is a general need in the art for a method of increasing the amount of data that can be transmitted over a bandwidth-limited communications medium, such as twisted copper pairs, without increasing the required bandwidth and thereby providing improved telecommunications services to the home and office.